// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//
// This file contains an implementation of an H264 Annex-B video stream parser.

#ifndef MEDIA_FILTERS_H264_BIT_READER_H_
#define MEDIA_FILTERS_H264_BIT_READER_H_

#include <stddef.h>
#include <stdint.h>
#include <sys/types.h>

#include "base/macros.h"
#include "media/base/media_export.h"

namespace media {

// A class to provide bit-granularity reading of H.264 streams.
// This is not a generic bit reader class, as it takes into account
// H.264 stream-specific constraints, such as skipping emulation-prevention
// bytes and stop bits. See spec for more details.
class MEDIA_EXPORT H264BitReader {
public:
    H264BitReader();
    ~H264BitReader();

    // Initialize the reader to start reading at |data|, |size| being size
    // of |data| in bytes.
    // Return false on insufficient size of stream..
    // TODO(posciak,fischman): consider replacing Initialize() with
    // heap-allocating and creating bit readers on demand instead.
    bool Initialize(const uint8_t* data, off_t size);

    // Read |num_bits| next bits from stream and return in |*out|, first bit
    // from the stream starting at |num_bits| position in |*out|.
    // |num_bits| may be 1-32, inclusive.
    // Return false if the given number of bits cannot be read (not enough
    // bits in the stream), true otherwise.
    bool ReadBits(int num_bits, int* out);

    // Return the number of bits left in the stream.
    off_t NumBitsLeft();

    // See the definition of more_rbsp_data() in spec.
    bool HasMoreRBSPData();

    // Return the number of emulation prevention bytes already read.
    size_t NumEmulationPreventionBytesRead();

private:
    // Advance to the next byte, loading it into curr_byte_.
    // Return false on end of stream.
    bool UpdateCurrByte();

    // Pointer to the next unread (not in curr_byte_) byte in the stream.
    const uint8_t* data_;

    // Bytes left in the stream (without the curr_byte_).
    off_t bytes_left_;

    // Contents of the current byte; first unread bit starting at position
    // 8 - num_remaining_bits_in_curr_byte_ from MSB.
    int curr_byte_;

    // Number of bits remaining in curr_byte_
    int num_remaining_bits_in_curr_byte_;

    // Used in emulation prevention three byte detection (see spec).
    // Initially set to 0xffff to accept all initial two-byte sequences.
    int prev_two_bytes_;

    // Number of emulation preventation bytes (0x000003) we met.
    size_t emulation_prevention_bytes_;

    DISALLOW_COPY_AND_ASSIGN(H264BitReader);
};

} // namespace media

#endif // MEDIA_FILTERS_H264_BIT_READER_H_
